Visual space perception in the intermediate distance range (2-25 meters) serves a multi-faceted function. It allows the observer to perceive the 3D shapes, sizes and locations of objects, orient in space and guides our actions. Yet, despite its importance in enabling the observer to interact in the natural environment, knowledge of the mechanisms underlying space perception in the intermediate distance is incomplete. The long-term objectives of this project are to understand the basic mechanisms of space perception and to discover how these mechanisms mediate human space perception and action. Pertaining to the ability to locate an object on the ground, previous work found that the visual system achieves this by constructing a representation of the ground surface and then uses it as a reference frame to localize an object on the ground. To localize a suspended object requires determining the relative position between the suspended object and the ground reference frame. Recent empirical studies have provided critical evidence for the notion of a ground-based space perception, and led to the discovery of several fundamental mechanisms for representing the ground surface. This progress has motivated the formulation of a more comprehensive theoretical framework for understanding how the visual system forms the ground-based perceptual space in the current proposal. The specific aims are to seek quantifiable empirical proofs to substantiate and advance the framework. The following three broadly defined issues will be investigated: A. The Perceptual Mechanisms Used by the SSIP to Represent the Ground and Other Surfaces B. The Role of Top-Down Processing in Space Perception C. Locating an Object that Does Not Have a Direct Surface Contact with the Ground The research will employ the psychophysical method to test human observers, predominantly in the real 3D environment. In addition, to complement the research performed in the real 3D environment, the proposed project will investigate space perception in the virtual environment by capitalizing on the virtual reality technology. This approach is not only useful for controlling the experimental stimulation, but will also yield insights into how one can improve the immersive quality of virtual scenes. Overall, the findings and insights gained from the proposed project will advance the science of human space perception in the intermediate distance range. It will also improve our understanding of the visual problems confronted by people with compromised visual functions. PUBLIC HEALTH RELEVANCE: Research of space perception in the intermediate distance is important because a significant proportion of our daily activity, such as locomotion and navigating, are performed over this distance range. Besides its contribution to perceptual sciences, the knowledge gained from this project can provide insights into the space perception and action problems of patients with visual deficits (e.g., in strabismus and amblyopia), and the problems encountered by ordinary people in non-optimal visual environments. The approach of using the virtual reality technology in the proposed project has the potential of being adapted into a useful tool for aiding people with space perception deficits.